1. Field of the Invention
The present invention relates to a solid-state imaging device.
2. Description of Related Art
A technique is known which provides a monitoring sensor in a solid-state imaging device and controls a pixel output by using a monitoring sensor output, as described in Japanese Patent Application Publication (JP-A-Showa 59-196666). Such a technique is widely used in an auto-focusing image sensor. When the monitoring sensor is arranged in adjacent to an entire pixel sequence, a signal from the monitoring sensor reflects an average signal amount from the pixel sequence. By ending a charge accumulation of the pixel sequence when its output reaches a reference voltage, an approximately constant signal of the pixel sequence independent of light intensity can be obtained. At this time, when non-uniformity of the signal from the pixel sequence is great, a distance cannot be accurately measured sometimes since the signal is partially saturated even under such a control.
In the technique described in Japanese Patent Application Publication (JP-A-Showa 59-196666), the monitoring sensor is divided into a plurality of monitoring sensor sections and comparison with a reference voltage is carried out by a comparator for each monitoring sensor section. By calculating a NAND of outputs from the comparators, a minimum level of the signal is detected and the charge accumulation of the pixel sequence is controlled. The NAND result of the comparator outputs changes in response to the output change of a first one of the comparators. In response to this, a signal to read a charge signal from the pixel sequence is outputted and the charge accumulation is ended. In the technique described in Japanese Patent Application Publication (JP-A-Showa 59-196666), the charge accumulation of the pixel sequence is controlled by such a configuration and operation in relation with the monitoring sensor with the largest signal output amount (the smallest output level). For this reason, the partial saturation of the pixel sequence signal is hardly caused in comparison with a case of using a single monitoring sensor. In the technique described Japanese Patent Application Publication (JP-A-Showa 59-196666), minimum value is detected by arranging the comparator for each monitoring sensor and by calculating the NAND of outputs from the comparators. Accordingly, the comparators of the same number as the number of monitoring sensors are required.
The comparator is a relatively complicated circuit. Accordingly, an increase of the comparator leads to increase of a circuit area. When the number of monitoring sensors is increased to improve accuracy of the control, a control circuit becomes complicated to increase a chip size.